1. Field of the Invention
The present invention relates generally to variable displacement compressors. More particularly, the invention relates to an improved mechanism for coupling a journal to a rotary drive plate in a variable displacement compressor.
2. Description of the Related Art
Variable displacement compressors have a wide variety of applications including use as compressors for air conditioning and/or refrigeration systems such as automotive air conditioners. A conventional variable displacement compressor is illustrated in FIG. 6. In this compressor, a rotary journal 103 is coupled via a link pin 102 to a drive plate 101, securely mounted to a rotary shaft 100. A swash plate 104 is supported by the journal 103. A plurality of cylinders 105 are provided in a cylinder block. Each cylinder receives a piston 106 that is coupled to the swash plate 104 by connecting rod 107. The rotary motion of the journal 103 causes undulating movement of the swash plate which in turn drive the connecting rods and pistons in a linear reciprocating manner.
An arc shaped elongated hole 109 is formed in a support arm 107 that protrudes from the drive plate 101. The elongated hole 109 serves as a guide that slidably holds the link pin 102. This arrangement keeps the top clearance of the piston 106 approximately constant at the top dead center position regardless of the inclination of the journal 103 and the swash plate 104. Thus, the inclination angle of the swash plate can be controlled to vary the stroke of the pistons.
When the accuracy of the dimensions of the arc-shaped, elongated hole 109 is low, however, the top clearance of the piston 106 cannot be maintained nearly constant. Therefore, when the gap between the elongated hole 109 and the link pin 102 is large, noise is generated. At present it is difficult to further improve the accuracy of the dimension of the elongated hole 109.
The discharge pressure of the compressor is generally higher than the crankcase pressure. Therefore, when a piston is in its top dead center position, the pressure of the gas in the compression chamber (acting against the face of the piston) will typically be higher than the pressure of the crankcase gases acting against the back side of the piston head. This creates a resultant compressive force which acts against the swash plate 104 at a point of action Mf.
In this design, the point where the journal supporting pin 102 contacts the elongated hole 109 in the drive plate 101 will vary in accordance with the inclination angle of the swash plate. More specifically, as the inclination angle of the swash plate 104 decreases, the point of support Mk of the compressive force moves downward in the elongated hole 109, as shown in FIG. 6. At the same time, the point of action Mf on the swash plate 104 which receives the compressive force of the piston 106 that has reached the top position moves upward relative to the point of support Mk.
As a result, the point of action Mf of the compressive force is not aligned with the point of support Mk. Thus, the compressive forces produce a moment that acts to influence (in this case further reduce) the inclination angle of the swash plate 104. This moment destabilizes the control of the compressor which makes smooth control of the compressor's discharge capacity difficult.
Another prior art variable displacement compressor design is disclosed in Japanese Unexamined Patent Application Publication No. 61-149585. In that design a drive arm is secured to the drive shaft for rotation therewith. The drive arm has a C-shaped hinge arrangement that is pivotally coupled to a mating C-shaped member carried by the swash plate. The drive arm is mounted to the drive shaft in a position that defines the top dead center position of the piston. Crankcase pressure can then be used to control the inclination angle of the swash plate to control the compressor's displacement.
Since the drive shaft and the swash plate are directly coupled by arcuate pivotal hinges, any compressive load acting on the piston located in the top dead center position will be transmitted directly through the swash plate to the drive arm. Thus, the compressive load acting on the top dead center piston will not influence the inclination of the swash plate. Further, since an arcuate hinge arrangement is used, the point at which the compressive load is transferred from the swash plate to the drive arm will remain fixed regardless of the swash plates inclination. Although this design avoids the effects of the compressive load of the top dead center piston affecting the inclination angle of the swash plate, the C-shaped hinge members necessary to couple the swash plate and drive arm are relatively difficult to produce.